1. Field of the Invention
The present invention relates to a photoelectric conversion device, imaging system, designing method, and photoelectric conversion device manufacturing method.
2. Description of the Related Art
Photoelectric conversion devices such as a CMOS sensor are recently employed in 2D image input apparatuses such as a digital still camera and digital video camera, and 1D image reading apparatuses such as a facsimile apparatus and scanner. In the photoelectric conversion device, an object image is formed on a plurality of photodiodes arrayed two- or one-dimensionally. The photodiodes generate charges corresponding to light for generating an image signal. The photoelectric conversion device needs to increase the sensitivity of photodiodes.
According to a technique disclosed in Japanese Patent Laid-Open No. 2000-012822, an antireflection film is formed on each photodiode to decrease the quantity of light reflected by the surface of each photodiode. According to Japanese Patent Laid-Open No. 2000-012822, a plurality of photodiodes can efficiently generate charges corresponding to light, increasing the sensitivity of the photodiodes.
These days, photoelectric conversion devices require a larger number of pixels in a predetermined chip area. It is necessary to reduce an area occupied by the unit pixel. In some cases, photoelectric conversion devices need to be made compact. The chip area for forming a predetermined number of pixels is reduced by reducing the area occupied by the unit pixel.
When STI (Shallow Trench Isolation) element isolation portions are formed in a semiconductor substrate, the interval between a plurality of photodiodes can be decreased. With this arrangement, the area occupied by the unit pixel can be reduced, easily increasing the number of pixels in a predetermined chip area. Since the area occupied by the unit pixel can be reduced without reducing the area of photodiodes, the chip area for forming a predetermined number of pixels can be reduced.
As the area occupied by the unit pixel decreases, the size of the photodiode (photoelectric conversion unit) and that of the element isolation portion need to be further decreased. The size of the element isolation portion is restricted by a power supply voltage used in the photoelectric conversion device. It is more difficult to decrease the size of the element isolation portion than to decrease that of the photodiode. When decreasing the area occupied by the unit pixel, the shrinkage factor of the size of the photodiode sometimes becomes higher than that of the size of the element isolation portion. As the area occupied by the unit pixel decreases, the absolute area of the photodiode decreases, and a relative area occupied by the photodiode in the unit pixel may also decrease.
The present inventor has found the following problems. As the area occupied by the unit pixel decreases, the (absolute and relative) area of the photodiode decreases, and the interval between adjacent photodiodes sometimes decreases. In this case, the interval between lines arranged between photodiodes and color filters may also decrease. If the interval between lines decreases, light passing through it diffracts, increasing the ratio of light entering not the photodiode but the element isolation portion. Most of the light entering the element isolation portion is often reflected by the upper face of the element isolation portion. Alternatively, the light enters inside from the upper face of the element isolation portion, is reflected by the side or bottom face, and exits upward from the upper face of the element isolation portion. Not only does the area of the photodiode decrease, but also the ratio of light entering the photodiode decreases. As a result, the sensitivity of the photodiode may greatly decrease.